The inner ear receives an afferent innervation from ganglion cells and an effernt or centrifugal innervation from cells in the brainstem. Much has been learned about the organization of adult afferent and efferent innervation and the embryonic development of the vertebrate ear, particularly the mammalian cochlea. In contrast, very little is known about the embryonic development or the regeneration of the efferent system. The available evidence for a prenatal arrival of efferents at the developing hair cells allows speculation that they may play some role in the maturation of the efferent system. The long term objective of this proposal is to further our understanding of the role played by the efferent system in normal maturation and pathological defects of the mammalian cochlea. The first major aim of is to provide a detailed neuroanatomical analysis of the spatiotemporal development of the inner ear efferent innervation in embryonic mice, chickens, and frogs to estimate their potential role in normal maturation. The second major aim is to explore the interactions between the ear primordia and ingrowing efferent fibers involved in establishing the patten of efferent innervation. This will be done experimentally by exploiting the accessibility of non-mammalian embryos. The specific aims are: to separately delineate the sequence of arrival of afferent an efferent fibers at the inner ear epithelia, to determine the temporal sequence of the segregation of vestibular and cochlear efferent neurons, to determine the number of efferent neurons at various stages of development, to compare the developmental sequence of inner ear innervation in mice with that of chickens and frogs, and to determine the role of the developing ear in the proper pattern formation of efferent innervation and to establish the accuracy of regeneration of inner ear efferents. The proposed experiments will employ the lipophilic dye, Dil, and use the histochemical demonstration of acetylcholinesterase (the degradative enzyme of the major efferent transmitter) to delineate fiber pathways to and from the developing ear at various embryonic stages. Exploration of the mechanisms governing efferent fiber patterns during development and regeneration will be initiated by manipulating the ear primordia or eighth cranial nerve in non-mammalian embryos. These data should provide insight into ontogenetic mechanisms that lead to the specific patterns of efferent innervation in mammals, birds and frogs.